1. Field of the Invention
The present invention relates to electrical connection technology for connecting electrodes between substrates, and more particularly, to an electrical connection structure such as power supply wires to piezoelectric elements for driving ejection in an inkjet head, for instance, which are suitable for creating electrical connections between the terminals of wiring substrate and a plurality of electrode elements arranged two-dimensionally at high density, a liquid ejection head using this electrical connection structure, a method of manufacturing same, and an image forming apparatus.
2. Description of the Related Art
Japanese Patent Application Publication No. 2004-262190 discloses technology for connecting a flexible printed circuit (FPC) with piezoelectric elements by means of solder balls. In Japanese Patent Application Publication No. 2004-262190, solder balls with a core are used partially, thereby preventing the phenomenon of bursting of the solder and suppressing undulation (waving) of the FPC board.
Japanese Patent Application Publication No. 2004-136663 discloses technology for connecting a flexible printed circuit with the electrodes of piezoelectric elements, by using an anisotropic conductive paste (ACP) or a non-conductive paste (NCP), or the like.
Japanese Patent Application Publication No. 2003-25587 discloses technology for preventing disconnections between bumps and connection terminals, and shorting between adjacent electrodes, by forming insulating films of prescribed amount of projection between the bumps on a substrate, and electrically connecting bumps and connection terminals together inside the cavity (groove) between adjacent insulating films.
Japanese Patent Application Publication No. 2004-262190 relates to a method for creating direct electrical connections between the electrodes of piezoelectric elements and a flexible printed circuit, and it partially uses-solder balls with a core in order to ensure a space between the flexible printed circuit and the piezoelectric elements. However, in the case of this method, since the FPC board is soft, it is difficult to position the substrate during the assembly, and it is extremely difficult to ensure connection reliability with respect to a large number of electrodes in a two-dimensional matrix arrangement.
In Japanese Patent Application Publication No. 2004-136663, various electrical connection materials are used, but similarly to Japanese Patent Application Publication No. 2004-262190, substances which can act as bumps exist all over the place between the piezoelectric substrate and a flexible printed circuit, and since the FPC board is soft, then it is extremely difficult to position the substrate over a large surface area. Consequently, it is limited to use in cases where connections are formed over a relatively small surface area.
In order to resolve the problems associated with Japanese Patent Application Publication Nos. 2004-262190 and 2004-136663, Japanese Patent Application Publication No. 2003-25587 prevents shorting between mutually adjacent electrodes by positioning both of the pair of a connection terminal and a bump located between the two substrates, inside a groove (cavity) between the adjacent insulating films which are lower than the combined heights of the connection terminal and the bump, and it connects the electrodes reliably by filling an anisotropic conductive adhesive into the cavity.
However, the method described in Japanese Patent Application Publication No. 2003-25587 is only applicable to electrodes which are arranged one-dimensionally, and it is difficult to ensure good connection reliability for all of the electrodes in the case of electrodes arranged in a high-density two-dimensional matrix structure over a long and large surface area.